1. Field of the Invention
This invention relates to a position controller for controlling the pockets of a document sorter and more particularly to a multiplexed incremental position controller for simultaneously controlling a plurality of pockets.
2. Description of the Prior Art
FIG. 1 illustrates a typical encoder/sorter system generally at 10. The system 10 processes documents, such as checks and credit card receipts from left to right. Documents are placed in a feed hopper 12 by an operator, and then progress the length of the machine. After a document has passed a reader 14, an operator view station 16 and an encoder/endorser area 18, it is sorted to one of a plurality of pockets within a pocket area 20, based upon information collected during the processing of that document. As documents are sorted into the pockets, it is desirable to ensure that the bottom and leading edges of the documents are aligned, so as to facilitate subsequent operations. In order to obtain the desired degree of alignment, it is necessary to control the position of the stack of documents within each pocket.
FIGS. 2A and 2B shows the basic components of one pocket generally at 22. A document retainer 24 supports a document stack 26, and is connected to a D.C. stepping motor 28 by means of a rack and pinion gear (not shown). A document retainer stop 30 limits the maximum travel of the document retainer 24 to approximately 2.25 inches. A step-out sensor 32 and a step-in sensor 34 are each optoelectronic transmissive sensors utilizing LED's and silicon phototransistors. The mechanical position of the step-in and step-out sensors 32 and 34 is such that a minimum of twelve documents must enter a pocket to move a sensor actuator 36 from position 3 to position 1.
As a new document 38 enters the document stack 26, it positions itself between the last document to enter the stack and the sensor actuator 36. After a sufficient number of documents have entered the pocket, the sensor actuator 36 will be moved to position 1, breaking the optical path in the step-out sensor 32. A control system (detailed hereinafter) recognizes this condition and moves the document retainer away from the sensor actuator 36 (in the + direction) until a return spring 40 returns the actuator 36 to position 2. When an operator removes the documents 26, the return spring 40 moves the switch actuator into position 3, breaking the optical path in the step-in sensor 34. The control system recognizes this condition and moves the document retainer 24 toward the sensor actuator 36 (in the - direction), until the actuator returns to position 2. For proper operation of the pocket, it was empirically determined that the control system must respond differently to changes in the size of the document stack, depending on whether the change is a positive change (i.e. increasing size), or a negative change (i.e. decreasing size). An increase in the size of the document stack 26 is caused by a document 38 entering the pocket. This change is of a small magnitude (between 0.003" and 0.008" per document) and may be accompanied by switch bounce, due to the impact of the document on the actuator 36. Since documents generally move at a velocity such that they are spaced at least 100 milliseconds apart, and the switch bounce typically lasts 50 milliseconds, a control system should have a response time in the positive direction of approximately 75 milliseconds.
A decrease in the size of the document stack 26 is caused by an operator removing documents from the pocket. The change is of a relatively large magnitude and switch bounce is neglegible. Since it is desirable to restore the retainer to the stable position as quickly as possible, the control system should have a relatively short response time in the negative direction.
A conventional approach to controlling a plurality of pockets is to include a separate closed loop controller for each pocket. FIG. 3 shows such a controller generally at 42. A pair of sensor amplifiers 44 and 46 buffer the output of the sensors 32 and 34, and convert the outputs of the sensors 32 and 34 to a digital level. A direction and step rate logic circuit 48 combines the output of the sensors 32 and 34 with the signals from positive and negative step rate lines 45 and 47 respectively and outputs the appropriate direction and rate signals to a phase select logic circuit 50. The phase select logic 50 converts the direction and rate signals into a sequence of motor winding excitations, and a motor driver 52 converts the digital signals to the required power and voltage levels. A power reduction logic circuit 54 senses 100 milliseconds of continuous motor inactivity, and de-energizes all windings of the motor 28 during the remainder of the inactive period.
FIG. 4 shows the single step response of a typical stepping motor 28 in a lightly damped system. The time interval "A" represents the time required by the controller of FIG. 3 to determine that a step is required and to calculate and deliver the appropriate motor command. Once the command has been initiated, the controller 42 must wait for the stepping motor 28 to settle in its new position. Only after the settling is complete can the output of the sensors 32 and 34 be used to determine if additional steps are required. Consequently, during the single step response time, the controller 42 is idle. Therefore, to increase the hardware efficiency, a single controller can be utilized in a time-division multiplexed mode to control a plurality of pockets.